posix message queues and multiple receivers

What is the prupose or reasoning behind the fact that multiple processes
can open a message queue for reading using mq_open()?
I wrote simple mq sender and mq receiver programs; when I start multiple
receivers on the same mq, and send a message to it, only one of the
receivers gets the message, in a round robin fashion. That is probably
by design, but if a mq is meant to connect only two processes, why can
more than two processes open it?